1. Field of the Invention
The present invention relates to a servo control system, a lithographic apparatus including a servo control system and a method for controlling the position of an object.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In an embodiment of the scanner-type lithographic apparatus, the patterning device is supported by a patterning device support, which is movable in at least one scanning direction to move the patterning device through the radiation beam. The movement of the patterning device is carried out with high accuracy to avoid overlay errors during acceleration of the movable object
To control the positioning of the patterning device support with high accuracy, a servo control system is provided. The servo control system includes a measurement system to measure the actual position of the patterning device support. This actual position which may be measured in one or more directions/degrees of freedom, is subtracted from a set-point position of the patterning device support. The resulting error signal, i.e. the difference between the actual position of the patterning device support and the desired position of the patterning device support is fed into a controller unit which provides a control signal. The control signal is fed into an actuator, which actuator is configured to actuate the patterning device support.
In this known servo control system, it is assumed that during movements of the patterning device support, the patterning device does not move with respect to the patterning device support. However, it has been shown that with increasing accelerations of the patterning device support, there may occur slip between the patterning device support and the patterning device itself. As a result, the positioning of the patterning device support may correctly position a patterning device, when this patterning device is positioned in a nominal position, i.e. the assumed or initial position of the patterning device with respect to the patterning device support. However, when slip occurs between the patterning device and the patterning device support, the patterning device may not be correctly positioned with respect to the projection system although the patterning device is correctly positioned with respect to the projection system. Furthermore, the slip which may occur with a certain acceleration is not constant. Therefore, it may be difficult to predict the slip between the object and the movable support which will be obtained with a certain acceleration/movement of the movable support.